Jupiter

Jupiter taken by the Cassini spacecraft
from 16 million km on Dec 29, 2000.

Jupiter taken by Voyager 1 on March 1, 1979, from
a distance of 4.3 million km (2.7 million miles). The photo shows
Jupiter's Great Red Spot (top) and one of the white ovals that can
be seen in Jupiter's atmosphere from Earth.

Atmosphere

Jupiter's immense atmosphere consists of about 75% hydrogen and 25% helium
by mass (90% hydrogen and 10% helium by number of atoms), with trace amounts
of methane, ammonia, and other light substances.

The upper atmosphere is striated into wide parallel bands at different latitudes
because of a combination of the planet's rapid rotation and extensive convection caused by internal heat rising to the surface. Winds of more than 600 kilometers per hour
blow in opposite directions in adjacent bands, while slight chemical and
temperature differences between the bands are responsible for their different
shades of yellow, brown, orange, and red. The light-colored bands are referred
to as zones and the dark ones as belts. The zones are at a
slightly higher altitude and about 15 K cooler than the belts. Complex vortices
in the boundary regions between the bands were first seen by Voyager. The Galileo spacecraft's small descent probe
also found turbulence in the Jovian atmosphere, indicating that Jupiter's
winds are driven largely by the planet's internal heat rather than by solar
radiation as on Earth.

The colors of the surface gases are believed to be due in part to the release
of phosphorous and the formation of acetylene.
The colors correlate with the cloud's altitude: blue lowest, followed by
browns and whites, with reds highest. Sometimes we see the lower layers
through gaps in the upper ones. An enormous elliptical region in Jupiter's
South Equatorial Belt, known as the Great Red
Spot, is though to be a centuries'-old cyclone. Other similar but smaller
and less long-lived spots have been known for decades. Intense lightning
and powerful aurorae are other features of the Jovian atmosphere.

Rotation

Jupiter spins around on its axis once every 9.8 hours (Jupiter's "day").
This is so fast, given Jupiter's size and the fact that it mostly a fluid,
that Jupiter is noticeably squashed at its poles.

Measuring longitude values on Jupiter is complicated by the fact that the
planet spins more rapidly near the equator than it does at the poles. Three
systems are used. Jupiter System I is used for features within about
10° of the equator, where a full rotation takes about 9 hours 50.5 minutes. Jupiter
System II is used for features north and south of this zone (such as
the Great Red Spot), where a rotation takes about 9 hours 55.7 minutes. Finally, Jupiter
System III, which is based on the rotation of Jupiter's interior, is used
for radio observations and isn't particularly useful for visual observers.
This rotation time of 9 hours 55.5 minutes probably reflects the rate at which the solid
core of Jupiter rotates, far below the cloud layers.

Interior

Jupiter radiates about 2½ times more heat than it receives from the
Sun, indicating a substantial source of internal heat, almost certainly
gravitational contraction, to account for the surface temperature. At its
center, Jupiter is thought to have solid metal-rock core, similar in composition
to Earth, with a diameter of about 24,000 kilometers and a mass of 10 to 15 Earth-masses.
Surrounding this, out to a diameter of about 100,000 kilometers, is a metallic mixture
of hydrogen and helium. On Earth we know these two as gases; in Jupiter's
interior the pressure is so high that the hydrogen takes up a state in which
it behaves like a metal. Outside this metallic hydrogen zone is a shell
of liquid molecules, mainly hydrogen and helium, with the cloudy atmosphere,
richer in ammonia and methane,
about 1,000 kilometers deep, above. Temperatures range from -130°C at the top of
the clouds, to 30°C about 70 kilometers below, to perhaps 20,000 K at the planet's
center.

Magnetic field

Jupiter has a powerful magnetic field of about 4 gauss (the magnetic axis
inclined 15° to the rotational axis and about 0.1 Jupiter radius from the
center of the planet) and an immense magnetosphere that extends several million kilometers in a Sunward direction and more
than 650 million kilometers away from the Sun – past the orbit of Saturn!
In December 1995, the Galileo atmospheric probe discovered a new intense
radiation belt between Jupiter's ring and the uppermost atmospheric layers
that is about 10 times as strong as Earth's Van
Allen belts and contains high energy helium ions of unknown origin.

Other facts

Other measurements by the atmospheric probe confirmed that Jupiter and the
Sun have almost the same elemental composition and formed therefore from
the same primordial mixture in the solar
nebula. However, not all of the probe's measurements tied in with theoretical
predictions, the biggest surprise being the lack of water in the upper atmosphere.
Several ideas have been put forward to explain this finding. Perhaps water
concentrations vary with latitude, a suggestion that ties in with the observation
that most of Jupiter's lightning (an activity associated with water clouds)
occurs at middle latitude whereas the probe descended near the planets'
equator. Another possibility is that, during Jupiter's formation, water
tended to be confined to the planet's core by the massive overlying blanket
of hydrogen and helium.

Jupiter orbits the Sun five times further out than does the Earth, so that
each unit area of its surface receives only about one twenty-fifth, or 4%,
of the solar energy falling on a similar-sized patch of our own world. Although
Jupiter has virtually the same elemental composition as the Sun, it would
have needed at least 10 times more mass in order to have become a brown
dwarf, and about 80 times more mass in order to have initiated nuclear
reactions and shone as a star.

There has been some speculation about the possibility of life existing at
some level within Jupiter's atmosphere (see Jupiter,
life on). However, the main focus of astrobiological interest in the
Jovian system at present is the Galilean
satellites, most notably Europa.